Chromatographic Analysis of Gases from the Transformer the Process of Creating and Detecting Gases in the Transformers

EVENTSDIAGNOSTICS Photo courtesy of Končar D&ST, Zagreb, Croatia Zagreb, D&ST, Končar of courtesy Photo Chromatographic analysis of gases from the transformer The process of creating and detecting gases in the transformers 1. Introduction ABSTRACT Given the demands of today’s power systems, the question ince the beginning of production of oil for transformers of the transformer reliability is one of the top priorities. it has been shown that during their work, due to various The need for monitoring the performance of transformers Sphenomena, certain gases may appear [1]. In 1928, Buch- is imposed since the beginning of their use. This article holz relay was first used to collect gas bubbles passing through aims to analyse the impact of the gases released during the oil to the conservator. Due to the occurrence of gas in the breakdowns and energy discharge within the transformer transformer, the oil is gradually being displaced, which in a cer- that could accelerate the degradation of the insulation sys- tain amount pulls down the float and turns on a warning signal. tem and gradually lead to major faults and incidents. In larger amounts of gas, Buchholz relay trips the transformer. Consequently, there was a need for a quality analysis of the situ- ation inside the transformer in order to detect and eliminate po- KEYWORDS tential failures. In the early sixties gas chromatograph appeared, transformer, analysis, oil, gas, insulation a device used to identify gases dissolved in transformer oil. Since the seventies, this analysis is used to detect a number of different 36 TRANSFORMERS MAGAZINE | Volume 2, Issue 1 Emir ŠIŠIC Chromatography is a separation method in which components are separated and isolated in stationary and mobile phases of materials (out of which many are forming gases in the oil and above), damage to the insulation, reduction of operational safety, and ultimately failure or breakdown. Mineral transformer oil and cellulosic materials (paper) are usually used for the transformer insulation. Cellulose insulation gives the transformer dielectric strength and spacing between windings as well as the space between windings which are at a different potential. Mineral transformer oil, due to the impreg- nation in paper, increases its dielectric strength. The oil is mainly composed of saturated hydrocarbons linked by C-C and C-H bonds which crack as a result of faults with the formation of unstable parts, radicals or ions as well as other com- pounds. They are easily recombined in gas molecules, like in order by energy activation: hydrogen (H2), methane (CH4), ethane (C2H6), ethylene (C2H4), and acetylene (C2H2). Causes for forming gases can be classified into three categories: corona or partial discharge, pyrolysis (decomposition of subs tances under the influence of high temperatures), thermal de composition and sparks. Most of the energy is released during sparks, followed by overheating, and lastly due to the appearance of the corona. Gases that appear during fault are typical for de- gases. Seven of them are characteristic for degradation of transfor- gradation of the insulation system: hydrocarbons, carbon oxides mer oil-paper insulation: hydrogen (H2), methane (CH4), ethane and gases which do not come from failures [3]. Mentioned gases (C2H6), acetylene (C2H2), ethylene (C2H4), carbon monoxide (CO) begin to form at certain temperatures as shown in Fig. 1. and carbon dioxide (CO2). In addition, some gases (C3Hy) can also appear but they do not affect the work of the transformer much. Today chromatographic analysis of the transformer’s gases is one of the most important and the most sensitive methods for the early detection of changes in state of oil-paper insulation and thus represents an excellent indicator of the existence of potential failure [2]. Chromatography (derived from the Greek words chroma - colour and grafein - write) is the collective name for a group of techniques used for separating mixtures. It is a physical separation method in which the tested components are separated and isolated in two phases: stationary (fixed) and mobile (movable). Chromato- graphy as a laboratory technique can be: analytical (works with small samples and attempts to assess the contribution of individual components in the mixture) and preparative (separation of components from the mixture and a kind of purification). 2. Occurrence of gases in the transformer During the work transformers are exposed to electrical, thermal and mechanical stresses, leading to degradation of the insulation. Figure 1: The appearance of flammable gases at average temperature of The consequences are rapid chemical reactions and degradation oil decomposition www.transformers-magazine.com 37 DIAGNOSTICS Following the ratio of gases, it is important to consider the so- Modern gas chromatographs consist lubility of gases as a function of temperature. Fig. 2 shows that over the temperature range from 0 to 80 °C, solubility of some of the gas supply unit, the sampling gases increases above 79 %, while the solubility of other decre- unit with automatic injector, the co- ases below 66 %. lumn and the oven. It is equipped with temperature controllers and microprocessor systems Figure 3: Distribution between the two phases: A) equilibrium of molecules between stationary phases Figure 2: Solubility of gases as a function of temperature B) model of chromatographic process 3. Test methods tion of the two phases: fixed immobilised in a column or on a flat surface and movable, carrying the ingredients of the mixture Oil swabs are conditioned by constructional features of the trans- through the system. Different travel speed of individual compo- former. Well equipped devices have valves for sampling from nents will lead to separation (Fig. 3). In the initial phase the tested three levels of tank height as well as from the Buchholz relay and sample is introduced to a stream of inert carrier gas into a chro- the tap changer tank. It is important to emphasise that gases from matography column filled with solid or liquid stationary phase. the oil develop only when it is in operation, thus a quality repre- Liquid stationary phase may be absorbed on an inert solid sup- sentative sample should be taken while it is working, or soon af- port or can be immobilised on the walls of the capillary (capillary ter it has been shut down from service. column). The separation of mixtures will be carried out provided that they have different solubility in the liquid stationary phase. In case of sudden gas evolution, depending on the location and The first component that comes out is the one that is the least intensity of appearance, their distribution is uneven at first, but soluble in the stationary phase. equalises after a while. Therefore, in some cases it is necessary to take more samples of oil from different places immediately after Modern gas chromatographs consist of four units supported by the observed phenomena and again after a certain time. For con- temperature controllers and microprocessor systems [4]. The tinuous monitoring of results from the same transformer, it is im- first unit (gas supply unit) provides all the necessary gas supplies portant that samples are taken in the same way at the same place which may involve a number of different gases, depending on the and examined with the same test method. To examine and analy- chosen detector. Some detectors require hydrogen, air or oxygen se the gas composition, the sample must be taken from Buchholz to support combustion and a mobile phase supply that could be Relay or must be extracted from the oil. helium, nitrogen or some other inert gas. The second unit is the sampling unit with automatic injector with its own oven. The co- Testing is carried out using gas separation or chromatography. lumn represents the third unit, the important device that achieves Chromatography principle is based on equilibrium distribu- separation and an oven to control process temperature. Oven is used to maintain the temperature of the process at the optimum level and to improve separation of components. It should operate Gases from the oil develop only when over a wide temperature range (5 °C - 400 °C), but in practice the maximum oven temperature needed is usually less than 250 °C. the transformer is in operation, thus The oven often has air circulation driven by a powerful fan to en- a quality representative sample sure even temperature throughout the oven. The temperature in all parts should be stable at ± 0.5 °C.The fourth part of chromatograph should be taken while it is working, is the detector with wide range of available types. The output from or soon after it has been shut down the detector is usually electronically modified with data processing from service computer which processes the data and prints out report. After insertion of the components (mixed only with inert gas – mobile phase) and their separation at the exit of the column, 38 TRANSFORMERS MAGAZINE | Volume 2, Issue 1 they are placed in the detector. Detectors help visualising the Based on the results of gas analysis components eluted from the column. The obtained result after processing represents an input signal to the printer. The detector it is possible to diagnose three fun- registers a series of symmetric peak evaluations whose respon- damental causes of the insulation se depends on the concentration of ingredients in the sample which is recorded as a function of time. The position of the peak degradation which are divided into on time axis

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